In the last several years, minimally invasive surgical procedures, such as percutaneous transluminal coronary angioplasty (PTCA), have become increasingly common. A PTCA procedure involves the insertion of a catheter into a coronary artery to position an angioplasty balloon at the site of a stenotic lesion that is at least partially blocking the coronary artery. The balloon is then inflated to compress the stenosis and to widen the lumen in order to allow an efficient flow of blood through the coronary artery.
Following PTCA and other stenotic treatment procedures, a significant number of patients experience restenosis or other vascular blockage problems. These problems are prone to arise at the site of the former stenosis.
In order to help avoid restenosis and other similar problems, a stent may be implanted into the vessel at the site of the former stenosis with a stent delivery catheter. A stent is a tubular structure which is delivered to the site of the former stenosis or lesion and compressed against vessel walls thereat, again with a balloon. The structure of the stent promotes maintenance of an open vessel lumen. The stent can be implanted in conjunction with the angioplasty.
Stents can also be used to provide for local delivery of agents. For example, radiotherapy and drug delivery treatments applied to the site of the former stenosis following angioplasty have been found to aid in the healing process and to reduce significantly the risk of restenosis and other similar problems. Local delivery of agents is often preferred over systemic delivery of agents, particularly where high systemic doses are necessary to achieve an effect at a particular site. High systemic doses of agents can often create adverse effects. One proposed method of local delivery is to coat the surface of a stent with an agent.
A stent is typically coated with a primer layer and an agent layer. The primer layer is applied between the stent and the agent layer to improve adhesion of the agent layer to the stent. In some cases, the agent layer may be applied directly to the stent.
Spray coating is commonly used to apply a layer of coating to a stent. A spray coating apparatus typically includes a spray nozzle and a pump that supplies a coating substance from a reservoir to the spray nozzle. The coating substance is ejected through the nozzle to create a plume of coating substance.
During coating operation the stent is supported by a stent support, and the stent support and stent rotate about the axis of the stent support. The stent support is also configured to axially or linearly translate the stent through the plume of coating substance. The nozzle may be translated along the axis of the stent as an alternative to or in addition to axially translating the stent. The coating substance is deposited on the stent as the stent is translated through the plume of the spray nozzle from one end of the stent to the other end. After a selected number of passes through the plume, the deposited coating substance is allowed to dry or subjected to a drying process prior to further spraying of coating substance. The spraying and drying steps are repeated until a desired amount of coating substance is deposited on the stent.
The coating substance ejected by the nozzle is not uniformly distributed in the plume of the spray nozzle. The concentration of coating substance is highest in the areas along or near the longitudinal axis of the nozzle. As the distance from the axis of the nozzle increases, the concentration of coating substance decreases.
To increase the efficiency of coating operation, it is desirable to place the stent in an area of the plume that has a high concentration of coating substance, i.e., an area along or near the axis of the nozzle. To ensure that the stent remains in the desired area of the plume, it is important for the axis of the stent to be aligned with the axis of the stent support. If the stent support and stent are not coaxial, the stent will oscillate about the axis of the stent support during rotation, causing the stent to move in and out of the area of the plume with a high coating substance concentration. This will not only decrease the efficiency of coating operation but also produce an uneven coating pattern on the stent surface.
Additionally, misalignment between the stent axis and the stent support axis may cause inconsistent application of coating substance to the stents, with stents placed near the axis of the nozzle receiving more coating substance than stents placed relatively far from the axis of the nozzle. This variation in the amount of stent coating may increase the number of stents having coating weights outside of the acceptable range, thereby increasing the stent defective rate. These variations are difficult to compensate by adjusting the rate or duration of spray, because the misalignment is unpredictable.
Currently there are no efficient and reliable methods to ensure a proper alignment of a stent with a stent support.